1. Field of the Invention
The present invention relates to a method and a device for measuring the bleach requirement, bleachability, and effectiveness of hemicellulase enzyme treatment of pulp, and in particular, to a method and a device for rapidly making these measurements to enable pulp mill operators to determine more accurately the amount of bleaching chemicals required to bleach the pulp. The present invention also includes an optical device for use in the device.
2. Background of the Invention
Paper is made from wood and nonwood fibers. Wood is the predominant fiber source for paper in North America and Europe while straw predominates in Asia. The paper manufacturing processes based on wood and nonwood fibers are similar. The processes described below are for paper made from wood but can be directly related to paper made from nonwood fibers. This description is found in more detail in "Pulping Processes" by S. Rydholm, 1965.
The fibers used to make paper consist primarily of cellulose, hemicellulose, and lignin. The amounts of these three constituents present in the final paper product depends on the manufacturing process used. Pulp made from virgin fiber can be described in terms of the three primary classes of pulping processes: mechanical, semichemical, and chemical. Recycled pulp, also known as secondary fiber, consists of a combination of one or more of the three classes of pulp made from virgin fiber.
In mechanical pulping, the wood is broken up into a mat of pulp fibers by mechanical action, such as grinding with stones or blades or passing the pulp through refiners. When high temperatures are used to aid in the mechanical pulping, the process is known as thermomechanical pulping. Mechanical pulp has almost all of the original cellulose, hemicellulose, and lignin present and is used for lower-grade, disposable papers such as newsprint.
Paper made from mechanical pulp often must be of at least a minimum degree of whiteness to satisfy the end use. The degree of whiteness is known as the brightness of the pulp and is generally measured in terms of ISO brightness. One of ordinary skill in the art will recognize that there are various methods for measuring ISO brightness, such as TAPPI method T452. Mechanical pulp is sometimes bleached with perhaps 5 Kg/t hydrogen peroxide and/or 2 Kg/t sodium hydrosulfite to increase its brightness, although seldom to brightness levels obtained with other pulping methods. The bleaching chemicals are typically added in one operation for the hydrogen peroxide and a second operation for the sodium hydrosulphite. These bleaching operations are called stages.
Semichemical pulping refers to the combination of mechanical action and chemicals used to make pulp. The mechanical action is similar to that used in mechanical pulp. The chemicals added are typically neutral salts such as sodium sulphate. The presence of the salts aids in the pulping process and thereby decreases the intensity of mechanical action required. This decreases the damage to the fibers and results in pulp of higher strength than mechanical pulp. Semichemical pulps are used for products where some degree of strength is required, such as packaging. Like mechanical pulp, semichemical pulps retain almost all of the initial cellulose, hemicellulose, and lignin.
Semichemical pulps are sometimes bleached to improve their brightness. As with mechanical pulps, the most important bleaching chemicals are hydrogen peroxide and sodium hydrosulfite. The amount of these chemicals used is often 5-fold greater than with mechanical pulps, as a higher brightness is often desired. The bleaching of semichemical pulp is carried out typically with two or three hydrogen peroxide stages and then a sodium hydrosulfite stage.
The highest quality paper is made by chemical pulping processes. The manufacture of high quality, bright white paper largely depends on removing the lignin from the wood pulp with minimal degradation to the cellulose and hemicellulose. Complete lignin removal is essential for the production of fine paper, because lignin weakens and imparts color onto the pulp. The two principle chemical pulping processes are sulfite pulping, in which the pulping liquor is acidic sodium sulfite, and Kraft pulping, in which the cooking liquor is alkaline sodium sulfide. Kraft pulping is the more common process, with about 37 million tonnes of Kraft pulp and 3 million tonnes of sulfite pulp produced annually in North America.
In chemical pulping, 80% to 95% of the lignin is removed from the wood by cooking it in the chemical liquor in a batch reactor or a tall flow through tower, either of which is known as a "digester." After being washed with water, the cooked material contains 1.5% to 5% residual lignin and is known as brownstock. In some mills, almost half of the residual lignin is removed using an oxygen delignification reactor. Regardless of whether a mill carries out oxygen delignification or not, the remaining lignin is removed by a multistage bleaching process to obtain a bright, stable final product.
A general reference for bleaching pulp is "Pulp Bleaching: Principles and Practice" edited by D. Reeve and C. Dence, 1996 (hereafter "Reeve and Dence").
The first stage of a conventional bleaching process involves treating the brownstock with chlorine or chlorine dioxide or a mixture thereof. This is known as the "first stage" or the "chlorination stage", and the terms are used interchangeably herein. The chlorination stage is the most important bleaching stage, as 50% to 80% of the total bleaching chemicals are used in this stage.
In the second bleaching stage, the pulp is extracted with sodium hydroxide. This is known as the "second stage" or the "extraction stage", and the terms are used interchangeably herein. The chlorination and extraction stages reduce the lignin concentration in the pulp to less than 1%. The pulp requires 1 to 4 hours to pass through these two stages.
After extraction, the final lignin remaining in the pulp is removed in three additional stages. The "third" or "D1" stage consists of treating the pulp with chlorine dioxide. The "fourth" or "E2" stage is an extraction with sodium hydroxide. The "fifth" or "D2" stage is another chlorine dioxide stage. The final product of the D2 stage is the desired bright white pulp. Typically, 5 to 8 hours are required for the final three stages, and therefore 6 to 12 hours is required for pulp to pass through the bleach plant.
Secondary fiber can consist of paper made from mechanical, semichemical, or chemical pulping processes, or a mixture of paper made by any two or all three of these processes. The secondary fiber is repulped by processes analogous to, but milder than, the mechanical, semichemical, and chemical pulping processes used for virgin fiber. The secondary pulp is then bleached with hydrogen peroxide, sodium hydrosulfite, or chlorine-based oxidizing chemicals as is done with virgin pulps. A somewhat lesser amount of bleaching chemicals are required for bleaching secondary fiber than virgin fiber.
Whether the pulp consists of virgin or secondary fiber, and whether it is from a wood or nonwood fiber source, the most important specification for the bleached pulp is its brightness. The pulp customer specifies the minimum brightness that they can or will tolerate in their use of the pulp. Pulp that fails to achieve the minimum brightness, i.e. pulp that is underbleached, is known as "off-grade" pulp. This must be repulped and run through the bleach plant a second time, at great expense and loss of production time. Pulp whose brightness greatly exceeds the minimum for the customer is acceptable for sale, but the excess brightness is achieved at a cost of additional bleaching chemicals that is, in effect, wasted. This pulp is overbleached.
Pulp mill operators therefore balance between using a sufficient amount of bleaching chemicals to avoid off-grade pulp, but not overbleaching at large cost and waste of bleaching chemicals. Several methods and instruments have evolved to help the operators control the bleaching of the pulp. These are described in detail by Reeve and Dence.
The most common measurement used to estimate the amount of bleaching chemicals that should be added to the pulp is the brownstock Kappa number (hereafter "Kappa number"). The Kappa number is a measure of the amount of lignin in the pulp and therefore is related to the bleach requirement. Almost all bleached chemical pulp mills measure the Kappa number of the pulp entering the bleach plant.
The Kappa number is measured manually, semi-manually, or automatically. For manual measurement, a sample of pulp is oxidized by potassium permanganate, as described in TAPPI method T 236. This test is carried out by a technician and requires about 45 minutes to complete, including a separate step of drying and weighing pulp. The advantage of the manual test is that it does not require complex or expensive equipment. The disadvantage is the long time required and the variability in technique among technicians.
In a semi-manual determination, the technician prepares the weighed amount of dry pulp and places it in an instrument, which automatically adds the potassium permanganate and carries out the titrations required in TAPPI T 236. The instrument determines the Kappa number based on a titration, which is controlled by a spectrophotometer. Two commercial units of this semi-manual type are the Series SX 4400 Automatic Pulp Analyzers, from Systematix Controls Incorporated, Seattle, Wash., and KTS1 Titration System, from Radiometer America, Westlake, Ohio. The semi-manual method overcomes much of the variability associated with technicians, but is only slightly more rapid than the manual method.
Alternatively, the Kappa number is measured automatically by a Kappa number analyzer, several of which are commercially available.
The Kajaani Kappa Analyzer uses samples of pulp taken manually or automatically. For automated sampling, a sampling port takes a sample of pulp from the stock line and conveys it to the instrument chamber. For pulp of low consistency (less than 6%), the pulp sample is taken by opening a valve and allowing the pulp to flow out. At higher consistency, the pulp is pulled out of the line by the action of a piston. In either case, the pulp is conveyed along a line by flushing the line with cold water. Whether the sample is taken manually or automatically, once in the instrument chamber, the pulp sits over a mat screen and is washed thoroughly with water. The Kappa number of the pulp is then measured by a Xenon flash lamp with three detectors operating in the ultraviolet range. The STFI Opti-Kappa Analyzer works in a similar manner. It also conveys a 90 ml pulp sample from the stock line, in 4-6 minutes. The Kappa number is measured by using two UV detectors.
Automatic Kappa analyzers report data every few minutes, which is more rapid than the manual method. The drawback is the expense and maintenance associated with the instrument.
In addition to measuring the brownstock Kappa number, there are three common measurements of pulp as it enters the chlorination stage. These are (1) pulp consistency, (2) pulp brightness, and (3) residual bleaching chemical. These three measurements are carried out by almost every mill.
The pulp consistency is the solid content of pulp as it passes through the stock line. Clearly, the pulp consistency is an important measure of the amount of pulp present. Pulp consistency is measured and controlled by instruments sold by several companies, including Valmet Automation, BTG, and ABB. The instruments operate either based on the shear force delivered by the pulp fibers or by using optical measurements. The optical measurements may be of polarized light, such as the LC-100 instrument of Valmet. Alternatively, the optical measurements may be of infrared light, such as the TCA instrument of ABB. These pulp consistency measurements are continuous. The pulp brightness is measured by on-line instruments after the initial 30 seconds to 2 minutes of the chlorination stage. This pulp brightness achieved within the initial period of the stage gives an indication of the brightness that can be expected by the end of the stage. For the automatic detection of pulp brightness, several systems are available commercially. The BT-500 system of BTG uses four probes mounted on the pulp stock lines to measure the brightness of the pulp as it flows by. The probes operate at wavelengths of 480 nm, 560 nm, 650 nm, and infrared. Alternatively, the Pulpstar system of In-Line Sensors Inc. uses a tungsten-halogen lamp and two brightness detectors. On-line pulp brightness measurements are continuous.
The residual bleaching chemical is measured by on-line instruments after the initial 30 seconds to 2 minutes of the chlorination stage. The residual chemical remaining after the initial period of the stage is related to the Kappa number of the pulp and gives an indication of the chemical consumption that can be expected by the end of the stage. Automatic detection of residual chemical is carried out by on-line sensors such as a Polarox probe, which relates the amperage of the slurry to the concentration of bleaching chemical. On-line residual chemical measurements are continuous.
Further measurements are made at the end of the chlorination stage. These are usually manual measurements of pulp brightness and residual chemical and are primarily to confirm the readings of the on-line instruments. The manual measurement of pulp brightness is carried out by placing a dry pad of pulp in a brightness meter and detecting the brightness at a light wavelength of 457 nm, as described in TAPPI method T 452. The manual measurement of residual chemical is carried out by taking a sample of pulp liquor and titrating with sodium thiosulfate or other compounds to determine its concentration.
Some control measurements of pulp are made in the extraction and later stages. For pulp that has proceeded through the extraction stage, the most common measurement is of the Kappa number. This is carried out manually or automatically as described for brownstock. For pulp that has proceeded to the D1 stage, the brightness and residual chemical measurements are carried out as described for the chlorination stage. These measurements of pulp in these stages are useful in aiding the operators over the later bleaching stages by measuring the amount of bleaching chemical required in those stages.
In spite of the widespread usage of the various manual and automated measurements of Kappa number, pulp consistency, pulp brightness, and residual bleaching chemical, pulp mill operators still do not have a complete sense of the amount of bleaching chemical required to bleach the pulp. This lack of information causes the operators to overbleach pulp to achieve the brightness targets, at an added cost and waste of bleaching chemicals, or to overcompensate in decreasing chemicals, risking off-grade pulp.
The reasons for the shortcoming in the current control measurements is clarified by defining the terms bleach requirement and bleachability ( G. A. Smook, "Handbook for Pulp and Paper Technologists", p. 390).
Bleach requirement is the amount of bleaching chemical needed to bleach the pulp to a given level of brightness in a pulp mill. An example of bleach requirement is "30 Kg/t of chlorine dioxide bleaches a pulp to 90 brightness".
Bleachability is the bleach requirement of a pulp relative to its lignin content. The lignin content is often quantified by the Kappa number. One example of bleachability is in a comparison of two pulps with the same Kappa number: the pulp with the lower bleach requirement has a higher bleachability. As a second example, consider a mill that uses oxygen delignification to decrease the average lignin content of the pulp by 40% and decrease the average bleach requirement by only 30%. This use of oxygen delignification has decreased the bleachability of the pulp.
The concept of bleach requirement seems simple: if one knew the bleach requirement of pulp, one would just apply that much bleaching chemicals. Unfortunately, obtaining a direct measure of pulp bleach requirement has proven to be difficult. There are no instruments or methods for operators to use to do this. For such a method or instrument to be useful in a mill, it must (1) be an instrument of reasonably small size, (2) account for the effects observed in mill-scale pulp bleaching, without being confounded by artifacts created by working in a small scale, and (3) execute the measurements within a short time, perhaps 60 minutes, which is 5 to 20 fold shorter than the tilne required to bleach pulp in a mill. Small-scale measurements that do not scale up to the mill's operation are of little use to the operators. Information delivered over longer periods is not useful to the operators, as the properties of the unbleached pulp change with fluctuations in mill operations.
Several methods have been suggested to measure bleach requirement.
The laboratory-scale bleaching, otherwise known as "manual bleaching," of pulp is well known. Such bleaching is carried out by mixing the chemicals with pulp in Mason jars, plastic bags, or specially designed reactors such as the Quantum mixer, by Quantum Chemical Company, Twinsburg, Ohio, or the custom built titanium bleaching reactor at CPFP research in Hawkesbury, Ontario. These units and techniques enable the mill bleaching operation to be simulated in a laboratory. These methods and devices are therefore used to carry out bleaching at rates that match those in a mill, but not in the short times required for a bleachability assessment.
One device has been proposed to bleach pulp rapidly, the so-called BrineCell (BrineCell Inc., Salt Lake City, Utah). The BrineCell is a 2 liter reaction chamber filled with pulp and an aqueous solution of 10 to 40 g/L of sodium chloride. Electrodes at the base of the device convert the water and sodium chloride ions to a mixture of ozone, oxygen, hydrogen, chlorine, hydroxyl radicals, and sodium hypochlorite, which can oxidize the lignin and bleach the pulp. The process of generating the oxidizing products and bleaching the pulp is complete in 10 minutes. However, the BrineCell is not reported to simulate mill-scale bleaching of pulp, nor is the bleaching as carefully controlled as would be required to assess the bleach requirement of pulp.
The Roe chlorine number, TAPPI 253 om-92 (also called the Hypo number) has been used as an alternative to the Kappa number measurement. The Roe number measures uptake of chlorine in 10 minutes, rather than permanganate used in a Kappa number test. The Roe chlorine number extends the range of the Kappa number to include mechanical and semichemical pulps. However, the Roe chlorine number is no more related to pulp bleach requirement than the Kappa number.
Two mill-scale operations that save time in bleaching by neglecting washing can, in principle, be adapted to laboratory equipment. The first scheme that neglects washing of the pulp between bleaching stages is known as displacement bleaching (Reeve and Dence, p. 609). Unfortunately, displacement bleaching increases chemical usage relative to conventional bleaching and thereby does not simulate accurately conventional bleaching. A second approach to neglecting washing of the pulp is the Histed DnD bleaching sequence (Reeve and Dence, p. 385), which carries out a 5 minute chlorine dioxide stage and a 3 minute extraction stage without intermediate washing. However, although the Histed sequence carries out these two stages quickly, it is not useful for assessing pulp bleachability because it requires pulp to have passed through a chlorination stage and it requires a lengthy (4 hour) final D stage.
A third mill operation that is of some interest is the Papricycle wash stage (Reeve and Dence, p. 315). In Papricycle, chlorinated pulp is adjusted to pH 7 with alkine filtrate. The pulp is held at this pH and then adjusted to pH 10 for a conventional extraction. Therefore, Papricycle carried out on the laboratory scale does not save time in bleaching.
In the absence of a direct measure of bleach requirement, methods have evolved which measure bleach usage at a single stage, including chlorination stage measurements of pulp brightness and residual bleaching chemical. These methods are limited to addressing the pulp bleach requirement at the specific stage.
The other approach is to measure the lignin content, such as with the Kappa number, and use this to estimate the bleach requirement. This strategy leads to the concept of pulp bleachability.
A quantitative measure of the pulp bleachability is the molecular chlorine multiple, also known as the total Kappa factor (Reeve and Dence, p. 249), which is defined by Equation (1): ##EQU1##
In Equation (1), chlorine dioxide has 2.63 Kg equivalent chlorine per Kg ClO.sub.2, chlorine has 1.0 Kg equivalent chlorine per Kg Cl.sub.2, and hydrogen peroxide has 2.09 Kg equivalent chlorine per Kg H.sub.2 O.sub.2.
If all pulps had the same bleachability, then the TKf to bleach all pulps would be the same. A measurement of Kappa number would then accurately determine the amount of bleaching chemical required to bleach pulp. However, this is not the case. There are many factors that cause the bleachability of pulps to vary almost hourly within a mill. Among these factors are:
1. The differences in the nature of the lignin in the pulp, as Kappa number varies. The TKf is higher at high and low Kappa numbers than at intermediate Kappa numbers. For softwood Kraft pulp cooked in a conventional digester, the TKf is at a minimum at Kappa number 25 to 30 and increases outside this range.
2. The differences in nature of the lignin in the pulp among wood species. For example, oak is more difficult to bleach than aspen or eucalyptus. Mills change species frequently and are vulnerable to changes in bleachability because of it.
3. The variability in a given digester. The bleachability is influenced by all of the primary digester operating variables, including alkalinity, H-factor, and sulfidity.
4. The use of hemicellulase enzymes. This is described in more detail, as follows. Hemicellulase enzymes act on the hemicellulose portion of the pulp. Hemicellulose in pulp consists of two types of structures with polysaccharide backbones: arabinoxylan and glucomannan. The enzymes that have shown benefit in bleaching include xylanases, arabanases, and mannanases, with xylanases the most common in commercial applications at pulp mills (Yee and Tolan, Pulp and Paper Canada, October 1997). The most significant benefit obtained with hemicellulase enzymes is an increase in the pulp bleachability such that the TKf decreases by 8% to 15%.
Hemicellulase enzymes are usually added to the brownstock. In most mills, the washed brownstock is stored in a high density storage tower before being pumped into the first bleaching stage. The enzyme is added to the pulp as the pulp is pumped into the high density storage tower, and acts on the pulp as it is flowing through this tower. Typically 20 minutes to three hours elapse before the pulp exits the storage tower; the monitoring and control of this retention time is discussed in Foody, et al, U.S. patent application Ser. No. 08/568,516. Upon exiting the storage tower, the pulp is ready to be bleached. The enzyme treatment, by removing a portion of the hemicellulose in the pulp, makes the pulp easier to bleach.
Hemicellulase enzymes enhance the bleachability of pulp without significantly changing the Kappa number, brightness, or the rate of chemical consumption in the chlorination stage. Therefore, the effects of hemicellulase enzymes is not detected by conventional mill instrumentation.
In the absence of a measurement of the bleachability of pulp, the Kappa number gives an incomplete and inaccurate assessment of the bleach requirement. It would be desirable to have a measure of bleachability at a pulp mill. Such a measure would improve the control of the bleach plant by taking into account factors that effect chemical use that are independent of the Kappa number, and therefore not accounted for by present instrumentation.
In summary, for a variety of reasons, none of the above-described approaches has proven effective in rapidly assessing the bleach requirement and bleachability of pulp. Several methods have been proposed to address the specific issue of the action of hemicellulase enzymes on pulp. These methods could supplement existing instrumentation and include:
1. Measuring the sugars released from the pulp by the enzyme. For example, xylanase releases xylose from pulp, and the amount of xylose released correlates with the degree of enhancement of the bleaching of the pulp. However, the detection of xylose in a pulp filtrate is too time consuming to offer sufficient feedback to the mill operators.
2. Measuring the decrease in hemicellulose content of the pulp caused by the enzyme. For example, xylanase enzymes decrease the xylan content of pulp. However, analysis of xylan content is a lengthy procedure that is too time consuming for feedback to the mill operators.
3. Measuring the increase in chemical oxygen demand (COD) of the pulp liquor. Hemicellulase enzymes release soluble sugars and other compounds into the pulp liquor that contribute to an increase in the COD of the pulp liquor. However, the increase in COD caused by the enzymes is small compared with the natural COD in the pulp liquor. Therefore, the increase in COD is not a sensitive measure of the action of hemicellulase enzymes.
4. Measuring the increase in extractable lignin. Hemicellulase enzymes increase the alkaline extractability of lignin out of the pulp. However, the increase in extractability due to hemicellulase enzymes is small compared with the natural extractability of the lignin. Therefore, lignin alkaline extractability is not a sufficiently sensitive measure of the extent of enzyme treatment.
5. Measuring the residual enzyme activity in the pulp after enzyme treatment. This is the method taught by Freiermuth, et al, Canadian patent application 2,146,207. The shortcoming of this method is that it is a determination of the activity of the enzyme and not the effectiveness of enzyme treatment of the pulp.
In summary, for a variety of reasons, none of these five approaches has proven effective in assessing the extent or effectiveness of hemicellulase enzyme treatment at a pulp mill.
Therefore, there are no instruments or methods available to enable operators to know the bleach requirement, pulp bleachability, or effectiveness of enzyme treatment of pulp in a timely manner. In the absence of this information, the operators can overbleach pulp, which leads to excess bleaching chemicals and costs, or underbleach pulp, which produces off-grade product.